1. Field of the Invention
This invention relates to a wavelength-variable light source unit used in an optical measurement technical field, etc., and more particularly to a wavelength-variable light source unit that can control the wavelength more finely.
2. Description of the Related Art
High-density optical communications or dense wavelength division multiplexing (DWDM) for entering a plurality of light beams provided with different information for each wavelength into an optical fiber is realized. A wavelength-variable light source is provided for evaluating the optical characteristic of an optical fiber or any other optical element used with high-density optical communications (DWDM) as a measurement object. According to the wavelength-variable light source, the wavelength of output light can be set arbitrarily in a wavelength range of about 100 nm to 150 nm. On the other hand, this kind of wavelength-variable light source adopts a semiconductor laser (laser diode (LD)), but the LD has a nature that an originating wavelength drifts, and thus if the wavelength of emitted light is set fixedly to a constant value, the wavelength changes minutely.
Then, there is also provided a wavelength-variable light source wherein, if reference light for monitor is separated and taken out from emitted light by a photocoupler, etc., and is monitored with a wave meter, etc., and change in the wavelength is recognized, the change can be fed back into a wavelength varying mechanism under manual control, thereby calibrating the wavelength deviation.
However, with the wavelength-variable light source in the related art, the wavelength of emitted light is adjusted mechanically and thus there is a limit in finely adjusting the wavelength. That is, the wavelength varying mechanism in the wavelength-variable light source in the related art is made up of a pulse motor (or a servomotor, etc.,), a piezoelement, and the like and changes the wavelength of emitted light by mechanically adjusting the attitude of a diffraction grating, a mirror, etc., of a wavelength selection element. However, the pulse motor has a large resolution per pulse and is hard to finely adjust; this is a problem, and a backlash occurs in the mechanism driven by the pulse motor; this is also a problem.
A wavelength-variable light source of the type wherein a wavelength varying mechanism adopts a plurality of piezoelements and the expansion and contraction of the piezoelements are used to move a shaft loosely piercing the piezoelement, thereby changing the attitude of a diffraction grating, a mirror, etc., can guarantee the accuracy to some extent as compared with the wavelength-variable light source installing a pulse motor, yet minute looseness on the mechanical structure such as a stick slip occurs and the structure becomes very complicated; this is a problem. Thus, generally the wavelength accuracy of emitted light is not sufficiently guaranteed with the wavelength-variable light sources in the related arts.
It is therefore an object of the invention to provide a wavelength-variable light source having a simple structure and being capable of sweeping or calibrating a wavelength with finer accuracy.
The invention has the following features for solving the above-described problems. In the following descriptions, the components of the subject matter of the invention corresponding to those of the embodiment are referred to together with the numerals in FIG. 1.
According to the invention, there is provided a wavelength-variable light source comprising a semiconductor laser (101) with a non-reflection film (101a) given to an end face of the semiconductor laser and a wavelength varying structure (for example, lens 102, diffraction grating 106, mirror 106, arm 108, rotations haft 109, pulse motor 110, etc.,) for flexibly adjusting the wavelength of laser light generated by the semiconductor laser, characterized by controller (for example, wavelength adjustment section 30) for controlling the drive current value applied to the semiconductor laser based on the wavelength of laser light generated by the semiconductor laser.
The controller controls the drive current value applied to the semiconductor laser based on the wavelength of laser light generated by the semiconductor laser. The change rate of the wavelength of laser light generated relative to the drive current value varies depending on the physical characteristic of each semiconductor laser; for example, it is 0.3 [pm/mA]. Therefore, the accuracy of the wavelength of laser light generated by the semiconductor laser is guaranteed in this order and sweeping or calibrating the wavelength can be controlled with a finer resolution.
Since some components are added to an existing wavelength-variable light source having a wavelength varying structure for flexibly adjusting the wavelength of laser light generated by a semiconductor laser, the wavelength-variable light source of the invention can provide the advantage that rough wavelength adjustment, for example, in the 100-nm order is made mechanically with the wavelength varying structure rapidly and fine wavelength adjustment, for example, in the order of 0.1 [pm/mA] or less can be made electrically in the controller. This can reduce the manufacturing costs because the minimum improvement may be added to the existing wavelength-variable light source, and can also eliminate the need for installing a piezoelement in the wavelength varying structure, so that the structure of the wavelength-variable light source can be simplified.
According to the present invention, the wavelength-variable light source may comprise a wavelength setting section (for example, wavelength setting section 40) for setting a wavelength, wherein
the controller increases or decreases the drive current value applied to the semiconductor laser in response to the difference between the wavelength set in the wavelength setting section and the wavelength of laser light generated by the semiconductor laser, thereby matching the wavelength of laser light generated by the semiconductor laser with the setup wavelength.
When the operator sets a wavelength in the wavelength setting section, the controller increases or decreases the drive current value applied to the semiconductor laser in response to the difference between the wavelength set in the wavelength setting section and the wavelength of laser light generated by the semiconductor laser, thereby calibrating the wavelength of laser light generated by the semiconductor laser. Thus, if an error factor such as a drift of the semiconductor laser or a backlash or a stick slip of the wavelength varying structure occurs, the laser light having the wavelength set by the operator can be always stably be output with fine accuracy. Therefore, it is made possible to test the test object such as an optical fiber still more precisely.
According to the invention, there is provided a wavelength-variable light source comprising a semiconductor laser with a non-reflection film given to an end face of the semiconductor laser and a wavelength varying structure for flexibly adjusting the wavelength of laser light generated by the semiconductor laser, characterized by
a sweep section (for example, wavelength adjustment section 30) for sweeping the wavelength of laser light generated by the semiconductor laser by changing the drive current value applied to the semiconductor laser.
The sweep section sweeps the wavelength of laser light generated by the semiconductor laser by changing the drive current value, so that the wavelength can be swept more flexibly as compared with the related art wherein the wavelength is swept mechanically with a motor and a piezoelement. That is, in the system of sweeping the wavelength mechanically, the wavelength of laser light generated is swept only in one way either from short wavelength to long wavelength or from to short wavelength to circumvent the backlash problem. In the invention, however, the wavelength of laser light generated by the semiconductor laser is swept electrically and thus the backlash problem, etc., cannot occur. Therefore, it is made possible to sweep the wavelength of laser light generated in two ways and consecutively between the short wavelength and the long wavelength.
Here, the wavelength of laser light generated by the semiconductor laser can also be swept in accordance with any waveform such as a sine waveform. However, preferably the sweep section sweeps the wavelength of laser light generated by the semiconductor laser so that the wavelength of laser light generated by the semiconductor laser changes in accordance with a triangular waveform as in the invention as claimed in claim 4. In this case, the wavelength characteristic of an optical fiber, an optical cable, an optical device, or any other measured object can be measured in a short time.